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Mushroom Compost

by David Sarasua


   Mushrooms growing in compost

 Mushrooms in the store


One of the joys of living in the bay area is that this is one of the largest mushroom growing regions in the United States. Now you ask why is this a joy? If you live next to a mushroom farm, they produce a smell that can only be described as very unpleasant. Well, I love to garden and happen to have clay soil. Actually, I should say, that I had clay soil. I now garden in pure mushroom compost.

I am fickle when it comes to gardening. What I plant somewhere today, I usually will dig up and plant somewhere else in the future. I am not sure why I do this, but I do. I think I get a pleasure in dealing with the soil. Anyway, when I first bought the house, I thought if I would turn the soil and add a bit of gypsum, my soil would be nice and fluffy. I'm not sure where I heard this, but it was advice that I heard time and time again.

I borrowed a rototiller and wanted to turn 12 inches of soil. Well even the rototiller that I borrowed, a real beast, could only go down six inches. So I rototilled six inches and used a shovel to till the other six inches by hand. I did not know how much gypsum to add so I added about 100 bags to my back yard. It was not real expensive and I really wanted my soil to be fluffy. Boy, was I wrong. One good rain and the material would revert back to its same hard character.

A friend of mine was having his house torn down. He asked me if I wanted some of the plants, as they were all going to be replaced. My neighbor and I went over in my pickup truck and we proceeded to dig out these plants. This soil was the same clay that I had at one time. We broke two indestructible shovels from Home Depot. These where the fiberglass shovels that have a limited lifetime guarantee, limited to what you use them for and how you use them. When I bought the house, I had to dig a hole to mount a mailbox. It took about four hours to dig that hole. I finally borrowed an electric jackhammer. Gardening in clay is not a pleasure.

I really enjoyed gardening but this clay soil made what I enjoyed a real drudgery. I would donate my time to Saratoga Horticulture Research Foundation. They did not garden in clay, they used a potting mix. Now here is soil that urges you to garden, nice and fluffy! This is the way I wanted my garden soil to be. Now how to get my garden similar to the potting mix?

Driving around the neighborhood, I noticed these ugly raised beds in the parking strips. Why would anyone do such an ugly thing? Answer: the clay soil was so hard that it was easier to bring in six inches of soil than to deal with the material.

Well, getting back to mushroom compost.


Clay weighs about 2,000 lbs. per cubic yard and it has a high cohesion level; it binds with itself.

A friend of mine who operates a back hoe states that clay is considered bed rock for grading. I think having a house built on a clay foundation is great, but as a foundation for gardening, I was not pleased.

   A squeezed lump of clay soil Mushroom compost weighs 500 lbs. per cubic yard and has a much lower cohesion level.

There is a simple pie pan test where you take the material in your garden and mix it with any amendment to determine how much of that amendment you should add. Take ten pie pans and mix your soil with an increasing 10% amendment mixture (the first one is 10%, the second is 20% and so on). Take this mixture and mix with water and let it dry. When you get a mixture that makes you happy, then you know how much to add to your garden. Now when I did this, it was not until I got to 50% before I was satisfied. Adding 50% of some amendment to my garden was not going to be an easy task. Think about this. I remember reading somewhere that the roots of most plants reside in the first twenty four inches of your soil.

What I wanted to do was to completely remove the first twelve inches of soil and then mix the second twelve inches with some type of amendment. Economically this is difficult. I found it was cheaper to just remove all twenty four inches and put in a potting mix. Since potting mixes ranges from about $20 to $40 a cubic yard, this was not a cheap solution. I needed a more affordable solution.

I read all I could find on soil and was amazed at the science. Did you know that loam is a combination of sand, silt and clay? In all my research on soil, I found that a number of potting mixes contained a material called mushroom compost. Why were they using this product and why was it relatively inexpensive when compared to other mixes? The reason is that it is a waste product of the mushroom farms in our area. Mushrooms, actually mycelium, need organic material to colonize and produce mushrooms. After the mycelium breaks down organic material and reproduces the material is spent as far as the mushroom farm is concerned. However, this is the stage where the plant material loves the waste product. This happens in nature all the time. Fungus breaks down organic material and the plants use the material to grow.

Not knowing about this material, I decided to learn more. There was a lot of information on mushroom compost. A lot of it wrong. Just like the advice I got on adding gypsum to the garden.

Some people stated that the material was salty and hot. Now salty or hot was not the same when you talked to different people. I thought salty meant table salt and hot meant heat. When you asked people to explain what they meant by salty or hot a lot could not explain. It was something they heard but did not understand.

All fertilizers are salts. Salts are a type of chemical bonding that are unique. They are a weak bond and come apart when in solution form. For example, if you take water, specifically deionized water and test for electrical conductivity, you will find that water is not a very good conductor. We all know that water conducts electricity. Salt dissolved in water does conduct electricity.

Plants need fertilizers but are limited in how much they can deal with at any one point in time. When you first see a soil analysis, you wonder about these terms such as pH, EC and CEC. The first time I saw this, I thought you had to be a scientist to read this stuff but it really is not difficult to understand. EC stands for Electric Conductivity. This is how they determine how much salt is in the material. Most plants can only tolerate an EC under 4. I was surprised to find that the EC was about 14 for most soil amendments.

Let's talk about pH for a minute. Most plants can not take up nutrients unless the pH is between 6.5 and 7.5. If the pH gets outside this range, the plant cannot absorb the nutrients. CEC stands for cat ion exchange capacity. Soil is negatively charged. What holds the nutrient is this charge. Most nutrients are positively charged. Some have more voltage than others and the higher the voltage the stronger they can hold on to the soil particle. Nitrogen is a very weak charge and is easily displaced by other nutrients. This is the reason that most soils are deficient in nitrogen.

All soils are not equal. Some soils have a higher negative charge than other soils therefore they can hold onto more nutrients. One of the reasons that organic material is a blessing to gardeners is that organic material has a stronger negative charge.

This is an article on mushroom compost so let's get back to it. I went down to a rockery and purchased some material and really enjoyed running my hands through it. I smelled it and it had an earthy smell. Well it turns out that there is a life form called Actinomyces. It is suppose to be part bacteria and part fungus, the scientific community in classifying fungus and bacteria did not know where this life form belongs. This life form excretes a smell that is very earthy.

After much research on mushroom compost and soil, it turns out that given certain parameters, a soil scientist can tell you if a plant will do well in a particular material, but you need to actually try the material to determine if it really can do what they say. So I planted in varying mixes of mushroom compost. Mushroom compost is sort of like wine, in that wine is a living breathing medium. I found that when I took a fresh batch of material to the soil lab, it had an EC of 14. A year later, the same material would have an EC of 6. What happened? One of the nitrogen sources that this mushroom farm would use was DPW, Dried Poultry Waste, a very nice term for chicken waste. In that particular waste was Potassium Chloride. Potassium Chloride has a low charge and is extremely water-soluble. One good rain and the medium from the stand point of EC, was very well close to what plants can tolerate.

Anyway, I tried growing in varying mixes of mushroom compost. The plants that I tried were agapanthus, vinca, onions, roses and geraniums. They all did rather well.

I was talking to a backhoe operator who gave me some prices to do some mixing. It was cheaper to dig out and replace the soil than it was to mix. So I crossed my fingers and had a part of my parking strip dug out and replaced with mushroom compost. We dug three feet down and backfilled with pure mushroom compost. After a year, I found this part of the garden was the best to garden in...the soil was a pleasure to work with.

My wife thinks this is the man's approach to gardening as she stated that she would have constantly amended the soil. The problem with this as with the computer that I use she has her own computer but insists on using mine is that she likes to garden in the spot that is pure mushroom compost.

My last thought on this is that mushroom compost is not the same material everywhere. Also, mushroom compost has very little to do with mushrooms except that it is the medium that is used to grow mushrooms. In understanding what a fungus is, it is a life form that decomposes organic material. After the fungus does what it needs to do, the plants just love the material. The term mushroom compost that we are discussing is called by many names. The farms called it their spent material.


Western Horticulture Society
PO Box 166, Mountain View, CA 94042
(650) 948-4614 or (650) 941-6136